It should be noted that there are two types of sirloin roasts....the Top Butt Sirloin and The Bottom Sirloin. Both would do well as as a slow roasted beef at 225*. You could sear at the beginning or at the end. Using the low and slow approach, you could expect 25-30 minutes per pound, or about an hour to reach 122-125* for medium rare. Hold for an hour and you will have a great little roast. If you do not have the two hours time, them I suggest the reverse sear method at the higher temperature of 275*...bring it to about 105*, then sear both sides for a couple of minutes each to give it a nice char....Rest for at least 10 minutes in your oven at the lowest setting of 140* and it will not lose too much heat....again, the longer you rest, the better the meat.